Turbine engines, such as those used to power commercial and military aircraft, include a turbine module comprising a bladed turbine circumscribed by a case. The turbine includes a hub, which is rotatable about an engine axis, and a set of blades projecting radially from the hub. The blades span across a working medium flowpath so that the tips of the blades are separated from the case by a small clearance gap.
The size of the clearance gap can change during engine operation because the hub and blades respond to centrifugal force arising from rotation of the hub whereas the case does not. In addition, the hub, blades and case all respond to temperature changes but at different rates. Despite these differences in mechanical and thermal response, it is nevertheless important to control the size of the clearance gap. If the gap is too small, a rapid acceleration of the engine could cause the blade tips to contact the case, resulting in damage to the blades or case. If the gap is too large, the efficiency of the turbine suffers.
Engine designers have devised various ways to control tip clearance. A well known arrangement uses cooling air ducts circumscribing the engine case. The ducts are radially offset from the case leaving an annular space between the duct and the case. A modulating valve commanded by a controller regulates the admission of relatively cool air into the duct. Coolant outlet holes penetrate the radially inner side of the duct. When the controller commands the valve to a fully or partially open position, cool air enters the ducts, discharges through the outlet holes and impinges on the case. This cools the case, causing it to contract radially toward the blade tips, the amount of contraction being related to the temperature and quantity of cool air admitted to the duct by the valve. When the controller commands the valve to close, the flow of cool air ceases, allowing the case to expand radially away from the blade tips. In order to achieve the desired tip clearances the designer establishes one or more control “schedules” that the controller uses to command the position of the valve as a function of engine operating conditions such as flight condition (e.g. altitude and airspeed) and engine power setting.
One shortcoming of the above described system is that the designer must make estimates of the amount of cooling air required at a variety of operating conditions. In doing so, the designer must account for the inevitable inaccuracies in these estimates. In addition, the controller must work on all engines on which it might be installed, irrespective of manufacturing variations and differing rates of deterioration of the individual members of an engine family. As a result, most engines will operate at non-optimum tip clearance most of the time. Typically, the designer intentionally errs in the direction of having tip clearances that are too large rather than too small. This is because excessively small clearances can, under some conditions, result in the blade tips contacting the case and causing damage, whereas excessively large clearances merely result in poor efficiency.
One other way to control tip clearances is to program the controller with a mathematical model of the engine so that the controller can continually estimate the existing tip clearance as a function of engine operating condition. This allows the designer to use a closed loop controller that minimizes the error between the estimated clearance and a desired clearance. However the mathematical model can also suffer from inaccuracies that lead the designer to err in the direction of excessively large clearance. In addition, the model imposes additional computational demands on the controller.
Another system is described in U.S. Pat. No. 4,330,234 which shows a probe with an open end exposed to a turbine flowpath at a location radially outboard of a row of blades. The probe is connected to a supply of pressurized air. Each time a blade passes the probe, the pressure in the probe is affected depending on the smallness of the blade tip clearance. The effect sensed by the probe is apparently an intermittent pressure pulse superimposed on the pressure of the supplied air. The patent does not reveal the relationship between the magnitude of the pulse and the tip clearance, nor how the fidelity of that relationship might be affected by changes in flight condition, engine power setting and probe supply pressure. Despite the presumed merits of the disclosed system, it clearly adds weight, cost and complexity to the engine. Moreover, if the pressurized air fed to the probe is extracted from elsewhere in the engine, the engine will experience a loss of efficiency, not unlike the efficiency loss that the clearance control system seeks to remedy.